1. Field of the Invention
The present invention is directed to a pressure plate for a friction clutch, wherein the pressure plate has at least one friction surface region which can be pressed against a friction facing arrangement of a clutch disk or the like, and wherein the pressure plate is formed at least partially from a first material, and wherein a second material having thermal contact with the first material is provided in the pressure plate at least in some areas.
2. Description of the Related Art
In friction clutches, pressure plates serve to clamp the friction facings of a clutch disk between a flywheel and the pressure plate. In this way, a torque transmission state is produced when the clutch is closed. In order to achieve this result, the pressure plates are acted upon by an energy accumulator, for example, a diaphragm spring, and are pretensioned in the direction of the friction facings and flywheel. During the engagement and disengagement processes, the different surface regions of the friction facings and pressure plate or flywheel enter into frictional engagement with one another resulting in friction heat.
Particularly when the friction clutch is used in a vehicle, heat is guided to one side of the pressure plate on its friction surface during the startup process. The flywheel is also heated on one side, so that the effects, disadvantages and steps described in the following can also apply to the flywheel.
Because of the heating on one side, a temperature difference occurs relative to the other side of the pressure plate which faces away from the friction surface. This temperature difference initially causes an elastic deformation of the pressure plate such that the radial outer area of the pressure plate draws back from the friction surface, so that the average friction radius decreases. This is known as xe2x80x9cumbrella deformationxe2x80x9d. This umbrella effect causes the pressure plate to deform in the manner of an umbrella such that its radial inner area is pressed in the direction of the flywheel, which means that this area can protrude in axial direction with respect to the radial outer area of the pressure plate.
However, this impairs reliable transmission of the clutch, because the latter can assume an unwanted slip state. Further, during subsequent cooling, the radial outer area of the pressure plate can remain deformed, so that the friction facings wear in a conical manner (usually more on the radial outer side than on the radial inner side).
Previously, in order to counter this problem the pressure plate was made as thick as possible. However, this resulted in a disproportionately heavy and expensive pressure plate. Further, a large space requirement resulted. In another suggested solution, the pressure plate was provided with a concave shape in the cold state. This solution also has a number of disadvantages, for example, inhomogeneous support in the cold state. Another suggestion consisted in providing the pressure plate with a ventilation system for cooling. This solution has the disadvantage that the pressure plates are usually produced from gray cast iron and that there is a poor transmission of heat between gray cast iron and air. Another grave disadvantage is that cooling begins too late to prevent umbrella deformation. It has also been suggested to produce the clutch with a greater diameter. However, this solution has the disadvantage that clutches with large diameters are heavier and more expensive and also have a large space requirement. Further, a larger clutch has an excessive mass moment of inertia, which in turn impedes engine acceleration.
In order to prevent negative effects of this kind which are induced by increased temperature in the friction region, U.S. Pat. No. 6,202,820 suggests a pressure plate for a friction clutch which initially has at least one friction surface region which can be pressed against a friction facing arrangement of a clutch disk or the like. In the friction surface region, the pressure plate is formed at least partially from a first material which ensures that the heat generated by friction is quickly carried away. In a body region adjoining the friction surface region, the pressure plate has a second material which has a high heat absorption capacity for the heat carried away from the friction surface region.
The solution described in the patent ensures that sufficient energy is conducted from the region contacting the friction facings during the engagement and disengagement processes which generally only take up very short periods of time (this means that the generation of heat energy or the conversion of kinetic energy into heat energy will also take place only for a very limited period of time). This energy is occasionally stored temporarily in the body region and is then guided away from the body region to the outside.
The object of the present invention is to provide another solution by which deformation of the pressure plate caused by heat can be prevented in a simple yet dependable manner.
A pressure plate for a friction clutch is suggested, wherein the pressure plate has at least one friction surface region which can be pressed against a friction facing arrangement of a clutch disk or the like, and wherein the pressure plate is formed at least partially from a first material, and wherein a second material having thermal contact with the first material is provided in the pressure plate at least in some areas. According to the invention, the second material has a different thermal expansion, particularly a greater thermal expansion, than the first material.
In this way, umbrella deformation of the pressure plate under thermal loading can be prevented. For this reason, the pressure plates are constructed so as to be thinner, which is a considerable advantage with regard to cost and the installation space requirement. Further, optimum dependability of transmission of the clutch is ensured at all times.
Due to the second material with a different coefficient of expansion, which will also be referred to in the course of the following description as counter-expansion mass, the friction radius is increased, or its reduction is prevented, over several successive starting processes. The design can be realized in a neutral manner with respect to installation space. Further, common mechanical engineering methods and materials can be used.
A basic idea of the present invention is that a region of the pressure plate is constructed from a second material (e.g., aluminum) which has a different, preferably greater, thermal expansion coefficient than the first material, e.g., gray cast iron, from which the pressure plate is made in part.
In U.S. Pat. No. 6,202,820, frictional heat is guided away as quickly as possible and stored temporarily so that deformations are prevented. For this purpose, the first material has a high thermal conductivity. The friction surface region made from this material is in frictional contact with the friction faces of the clutch disk during the engagement and disengagement processes. Due to the heat-conducting material of the friction surface region, the heat is quickly conducted away into the second material which is in thermal contact with the first material. The second material comprises a material with high specific thermal capacity. Accordingly, it is suitable for storing heat in a particularly effective manner without resulting in negative deformation. The second material must be suitable for intermediate storage of the large quantities of heat occurring briefly during the engagement and disengagement processes without resulting in deformations in the pressure plate. At the conclusion of the engagement or disengagement process, the heat can be given off to the outside.
In the solution suggested according to the invention, deformation due to the occurring friction heat is not eliminated. Instead, the effect achieved by the second material is that a counter-expansion occurs which is controlled via the arrangement, mass and construction of the second material in such a way that it counteracts the deformation of the first material. In this way, the friction radius of the pressure plate is not changed as a whole or at least is not reduced. Accordingly, the second material is a counter-expansion mass relative to the first material.
In principle, the invention is not limited to determined materials for the second material. It is important only that the second material is made to expand when heat energy is absorbed such that the disadvantageous umbrella deformation of the entire pressure plate is reliably prevented.
The second material can advantageously have a greater coefficient of expansion than the first material. However, it is also possible for the second material to have either a negative thermal expansion coefficient or, in comparison to the first material, a smaller thermal expansion coefficient in terms of amount. In this case, the umbrella deformation is prevented or compensated in that the second material contracts when heat is introduced and generates a bending-back moment in a corresponding arrangement.
Gray cast iron, steel and the like, for example, can be, but need not exclusively be used as first material. Aluminum, for example, can be, but need not exclusively be used as second material. Aluminum is relatively inexpensive and has a thermal expansion coefficient which, at 24*10xe2x88x926/K, is roughly twice that of gray cast iron (=10xe2x88x9212*10xe2x88x926/K). However, magnesium (=26*10xe2x88x926/K), copper (=17*10xe2x88x926/K), zinc (=39*10xe2x88x926/K) or other metals are also possible. A further advantage of the above-mentioned materials consists in that they quickly absorb friction heat and expand quickly compared with gray cast iron because of their higher thermal conductivity. The high thermal conductivity can also be utilized in deliberate fashion for improvement in guiding heat away from the friction surface.
The mass distribution between the first material and the second material is given depending on need and application and depends, for example, on the contour of the pressure plate and on the resulting thermal expansion behavior.
The pressure plate can advantageously be constructed substantially annularly.
Various possible constructions of the pressure plate according to the invention for preventing negative umbrella deformation will be described in the following. Depending on the kind of counter-expansion to be achieved by the second material (counter-expansion mass), the second material can be provided in the pressure plate in different ways.
The second material can preferably be provided in the form of at least one added element in the pressure plate. Non-exclusive examples for suitable added elements will be described more fully in the course of the description.
In a further development, it is conceivable that the second material is constructed as a region of the pressure plate.
Depending on the embodiment form, the friction surface region of the pressure plate can be formed chiefly or completely from a first or second material.
The second material can be provided in the pressure plate in such a way that it expands, or can expand, primarily in radial direction. It is also possible for the second material to be provided in the pressure plate in such a way that it expands, or can expand, in axial direction. Naturally, a combination of the two features is also possible. By radial direction is meant a direction vertical to the principal axis of rotation of the pressure plate. By axial direction is meant a direction parallel to the principal axis of rotation of the pressure plate.
The second material can advantageously be provided at the radial outer circumference of the pressure plate. In a further development, the second material can, of course, also be provided radially inside the pressure plate. Of course, combinations of the above-mentioned features are also possible.
It has turned out to be particularly advantageous that a greater thermal expansion is brought about at the radial outer circumference of the pressure plate than at the radial inner circumference. Depending on the embodiment form, the radial component of expansion as well as the axial component of expansion are advantageous to compensate for offsets of the outer circumference of the friction surface.
In a further development, the second material can be provided, at least in some areas, on the side of the pressure plate which lies opposite the friction surface and which is not heated.
In order to place the second material in the pressure plate, the pressure plate can have at least one groove for receiving the second material. The at least one groove can be formed on the radial outer side and/or on the radial inner side of the pressure plate. It is also possible for the at least one groove to be provided on the side of the pressure plate which lies opposite to the friction surface and which is not heated. In this case, the area of the pressure plate located opposite to the friction surface could be cut out in a suitable manner, for example, and the second material could then be cast in this cut out region. Of course, any desired combinations and variants of the arrangement and quantity of grooves are also possible.
The at least one groove can preferably be inclined. Due to the inclination of the groove, the second material located therein, which has, for example, a circular ring shape which will be described in the following, is protected from falling out accidentally, for example, due to centrifugal force.
The second material can advantageously be constructed as at least one circular ring. This circular ring can be constructed as one or more parts. When the circular ring is constructed in multiple parts, there can be segments of 3xc3x97120xc2x0, 6xc3x9760xc2x0 or the like. When the second material is in the shape of a circular ring, it can be provided, for example, in a corresponding groove as was described above. The circular ring can initially be produced separately in one part or in multiple parts and can then be placed in the groove. The circular ring can also be cast in the groove. For purposes of fastening, the circular ring can be connected with the pressure plate so as to be fixed radially and, depending on the construction, axially, for example, by means of corresponding fixing elements such as suitable fixing pins or the like, by soldering (preferably when the second material is copper or zinc) and the like.
In a further construction, the at least one circular ring can have at least one pin-shaped projection which protrudes from the circular ring. This pin-shaped projection can likewise optionally be made of the second material.
In an embodiment form presented by way of example, the circular ring can be provided, for example, in a groove located on the side of the pressure plate which lies opposite the friction surface and which is not heated. The circular ring of second material can be inserted into this groove as a counter-expansion mass, as it is called. The circular ring can have the pin-shaped projections at the side facing the friction surface of the pressure plate. These pin-shaped projections can be prevented from falling out by means of fixing elements, for example, a snap head or closing head. When the fixing element terminates flush with the friction surface, its end face participates in the friction process. The heat is conducted via the pin-shaped projection into the circular ring. This projection accordingly serves for faster conduction of heat to the second material and for its heating and, therefore, its expansion.
In a further construction, the pressure plate can have at least one bore hole. Preferably, the second material can be provided in the bore hole in the form of at least one added element. The added element can be, but need not exclusively be, constructed as a pin, bolt, screw or the like. However, it is also possible that the bore hole is cast with the second material. A plurality of bore holes are advantageously provided and are arranged so as to be distributed about the circumference of the pressure plate.
If required and depending upon application, the at least one bore hole can be oriented tangentially and/or axially and/or radially in relation to the pressure plate.
In an embodiment form presented by way of example, a plurality of bolts made of the second material are inserted tangentially into the pressure plate provided with corresponding bore holes and are prevented from falling out, for example, by caulking. It is also possible, alternatively or in addition, to introduce bolts axially into the pressure plate and to utilize their radial expansion exclusively. When suitably fixed, e.g., by soldering, caulking. fashioning the bolts as screws, or the like, their axial expansion can be utilized in addition.
The pressure plate can advantageously have at least one notch at its radial outer circumference in which the second material is provided at least in part. The notch can be wedge-shaped, for example.
For this purpose, the second material can be constructed, for example, at the outer radial edge of the notch, in the form of a web connecting the sides of the notch. The web can be constructed, for example, as a circular ring and can be inserted into the notch. This can be carried out, for example, by soldering, caulking, casting in a positive engagement, or the like. In this constructional variant, the web contacts only a small annular area at the pressure plate. However, since the pressure plate is notched more deeply, the radial outer area can bend elastically axially.
The second material can also advantageously be constructed in the form of at least one added element formed as a bolt which penetrates the notch axially and/or radially. In particular, when the bolts penetrate the notch axially and are arranged in the radial outer area of the notch and, therefore, of the pressure plate, an elastic expansion of the pressure plate is possible in this area.
By means of a suitable design of the notch and of the added element of second material penetrating the latter, it is possible for the pressure plate to be ventilated from the inside. For this purpose, the side walls of the notch and the added element define a type of ventilation chamber. In this way and due to the elements of the second material, dissipation of heat can be improved. For this purpose, the added element of second material can have, e.g., corresponding holes, vane-like raised portions and the like for transporting air.
In a further construction, the pressure plate can be formed of the friction surface region and a body region adjoining the latter axially. In this case, the pressure plate is divided into two material regions.
In a construction presented by way of example, the friction surface region is formed at least partially of conventional material, for example, gray cast iron or the like. The body region, on the other hand, can be formed at least partially of the second material and/or one or more added elements of second material. When heat is introduced into the friction surface due to the friction process, for example, during a starting process, a temperature difference occurs which initially ensures that the radial outer region of the friction surface region draws back, so that the friction radius moves radially inward and the dependability of transmission in the clutch is reduced due to the umbrella deformation effect. When heat is no longer introduced, the entire pressure plate and therefore also the body region is increasingly heated. The second material in the body region, expands to a greater extent than the first material, so that with appropriately selected placement of the second material a directed expansion can be achieved in the form of a restoring bending moment which counteracts and compensates for the umbrella deformation effect.
In two non-exclusive embodiment forms which are given by way of example, one or more circular rings can be provided in the body region, particularly on the side of the pressure plate which is located opposite the friction surface and which is not heated. In another construction, it is also possible for the body region to be formed of the second material in its entirety.
The friction surface region and the body region can advantageously be connected with one another via a groove connection. The groove connection which can be constructed, for example, as a dovetail-groove connection is used to improve the connection of the two regions.
In a further construction, the pressure plate can have a cutting edge for supporting an energy accumulator. An energy accumulator of this kind is, for example, a diaphragm spring of the friction clutch. The cutting edge advantageously has means for preventing friction wear or abrasion. For this purpose, the cutting edge can be suitably coated or hardened. It is also possible to place a wire ring in the knife edge, e.g., by caulking, so that it is prevented from slipping out and abrading. Of course, other measures for protecting the cutting edge against unwanted abrasion are also possible. Further, the cutting part can also be part of a facing wear arrangement. The facing wear arrangement can be, for example, a run-up ring or the like.
The contour of the region of the pressure plate comprising the first material can preferably be adapted to the contour of the region of the pressure plate comprising the second material.
In a further construction, it is possible that the second material is provided in a chamber provided in the pressure plate and at least partially fills the latter. The chamber can be limited on all sides by regions of the first material. In another construction, it is possible that the chamber is defined by regions of a first material and partially by regions of a third material. The third material can define the chamber at the side of the pressure plate located opposite to the friction surface and/or on the radial inside and/or on the radial outside. The third material, for example, can be in the form of the second material or in the form of a material differing from the first and second material. The selection of the third material depends on the manner in which the heat generated at the friction surface of the pressure plate is guided away and on the way in which a resulting thermal expansion in the second material is to be directed in order to counteract umbrella deformation.
The pressure plate can advantageously have a region of first material and an adjoining region of second material, wherein the dividing line between the two regions extends from the radial inside to the radial outside in relation to the pressure plate. In particular, the dividing line between the two regions can extend in such a way that more of the second material is provided on the radial outside than on the radial inside. The thermal expansion of the second material can accordingly be controlled in a precise and defined manner. As a result, the friction surface of the pressure plate does not draw back (umbrella deformation) because of the friction heat and the friction radius remains sufficiently large for reliable transmission.
In a further development, the pressure plate can have a region of first material and an adjoining region of second material, wherein the dividing line between the two regions extends axially with respect to the pressure plate, so that the second material is provided on the radial outside compared to the first material. Accordingly, it can be achieved in a particularly advantageous manner that the radial outer region of the pressure plate expands and umbrella deformation is accordingly counteracted.
The second material can advantageously be applied to or introduced into the region of first material by die casting. Accordingly, it is possible to prevent a large distortion in the structural component part itself. In order to be able to achieve die casting, it is advantageous to provide a corresponding sealing surface at the region of first material. Die casting can be realized in a particularly simple manner when the second material is introduced into a corresponding groove in the form of a circular ring with rectangular cross section. The region of first material should be heated prior to die casting in order to improve the connection between the first material and second material and to reduce internal stresses caused by heat.
The first material and the second material can be connected with one another by a material bond or engagement and/or by positive engagement. A material engagement can be, for example, but is not exclusively, soldering, gluing, welding, a penetration of the second material into the friction surface, or the like. A positive engagement which can be carried out as an alternative or in addition to the material engagement can be, for example, a corresponding waffle pattern, screwing, riveting, the second material engaging around the first material, and the like.
In a further construction, at least one receiving groove, especially an annular groove, can be provided on the radial outside for receiving at least one supporting element. The supporting element can be constructed, for example, as a supporting ring. This supporting ring can be produced, for example, by winding a high-strength fiber, for example, a carbon fiber, or a wire. In this way, it is ensured that when thermal expansion occurs the pressure plate is secured radially and only the radial inner area is lifted from the friction surface or deflected.
The different features for realizing the pressure plate according to the invention which were described above can be realized individually as well as in any desired combination.
In addition to the described features, it is possible to influence the umbrella deformation behavior of the pressure plate by means of a corresponding geometric shape of the pressure plate. By varying the radial thickness, axial length, material, adding interruptions in the inner and/or outer area of the pressure plate and the like, its strength ratio from the inner area to the outer area can be adjusted. In addition, a deformation depending upon the temperature gradient and, therefore, a change in the friction radius can be prevented by means of this adjustment.
According to a second aspect, the present invention is directed to a thrust plate assembly, particularly for a friction clutch in a motor vehicle. According to the invention, this thrust plate assembly has at least one pressure plate according to the invention as was described above.
According to a further aspect of the invention, a flywheel or a secondary mass of a two-mass flywheel is provided which, according to the invention, has the above-described features specific to the pressure plate.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.